Electrostatic discharge (ESD) protection has emerged as a significant challenge for semiconductor devices. ESD, originating from such sources as mechanical chip carriers, plastic chip storage devices, or human contact, can generate voltages that are many times greater than the design voltages of integrated circuits. For example, the human body can supply an electrostatic discharge of up to 4 kilovolts, which can be devastating to integrated circuits that commonly operate at voltages of less than 5V.
In light of the challenge posed by ESD, many semiconductor devices are now equipped with ESD dissipation features. However, in a typical silicon-on-insulator (SOI) device, the presence of a buried oxide (BOX) layer places limitations on the structures that can be used for ESD protection. In particular, the presence of the BOX layer in SOI devices complicates the implementation of vertical NPN or diode structures. Moreover, the poor thermal conductivity of the buried oxide causes thermal failure levels to be much lower in SOI devices than in analogous bulk devices.
In addition, an accurate temperature sensing has been a big issue the most recent micro processor application. Ideality factor of diode has been recognized as a key parameter in temperature sensing devices. The conventional lateral diode in SOI application showed high series resistance and poor thermal conductivity, causing a significant deviation from a diode ideality.
Despite the foregoing problems, diode structures have been built on SOI films that provide some ESD protection and temperature sensing. FIG. 1 illustrates one example of a conventional lateral diode 101 structure built on an SOI film. The diode comprises a buried oxide layer 103 upon which is disposed a device layer 105 that includes cathode 107 and anode 109 implants and an N-well 111. A polysilicon gate 113 is disposed over the N-well 111 and is electrically insulated from the N-well 111 by a gate oxide layer 115. The polysilicon gate 113 is bounded on each side by spacers 117, 119.
While lateral diodes of the type depicted in FIG. 1 provide some ESD protection, the protection they afford is typically lower than that achievable with comparable bulk devices. More recently, however, it has been shown that further improvements in ESD protection can be achieved through the use of vertical diode structures. Such structures have been found to offer improved ESD protection compared to their lateral diode counterparts, due to improved lattice temperature distribution. Vertical diode structures offer the further advantage of occupying less space than their lateral counterparts.
An example of a vertical diode structure is depicted in FIG. 2. The vertical diode 151 depicted therein comprises a substrate 153 within which is defined an N-well 155. Anode 157 and cathode 159 regions are implanted in the N-well 155 and are in electrical contact with anode 161 and cathode 163 electrodes, respectively. A buried oxide (BOX) layer 165 is disposed over the N-well 155 adjacent to the anode 157 and cathode 159 regions. A shallow trench isolation (STI) layer 167 is disposed over the BOX layer 165 in the area between the anode 157 and cathode 159 regions, and an SOI layer 171 is disposed over the BOX layer 165 elsewhere. An interlayer dielectric (ILD) 173 is disposed over the substrate 153 in the vicinity of the anode 157 and cathode 159 electrodes and over the SOI layer 171 and STI layer 167.
While vertical diode structures of the type depicted in FIG. 2 do have some desirable attributes from an ESD protection perspective, they have two significant disadvantages. One is relatively low packing densities. This is a significant disadvantage in light of the ongoing trend in the semiconductor industry towards further miniaturization. The second is difficulty in formation of N-well 155. Since the N-well is under STI 167 and Box 167, it's not so easy to make it in conventional CMOS process. There is thus a need in the art for methods and devices which address the aforementioned infirmities. In particular, there is a need in the art for SOI devices having ESD and/or temperature sensing structures, and for methods for making the same, which have higher packing densities. There is further a need in the art for such SOI devices that exhibit thermal failure levels that are comparable to those observed in analogous bulk devices. These and other needs are met by the devices and methodologies described herein.